Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems related to seismic exploration and, more particularly, to mechanisms and techniques for providing a seismic source having bending parts so that it can achieve various depth levels.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of structures under the seafloor is an ongoing process.
During a seismic gathering process, as shown in FIG. 1, a vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the ocean surface 14, or may have spatial arrangements other than horizontal, e.g., variable-depth arrangement. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward, toward the seafloor 20, and penetrates the seafloor until, eventually, a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by receiver 11 on streamer 12. Based on this data, an image of the subsurface is generated.
In an effort to improve the resolution of the subsurface's image, an innovative solution (BroadSeis system of CGGVeritas, Massy, France) has been implemented based on broadband seismic data. The BroadSeis system may use Sentinel streamers (produced by Sercel, Nantes, France) with low noise characteristics and the ability to deploy the streamers in configurations allowing the recording of an extra octave or more of low frequencies. The streamers are designed to record seismic data while being towed at greater depths and are quieter than other streamers. Thus, the receivers of these streamers are best used with a marine broadband source array.
A marine broadband source array may include one or more sub-arrays (usually three sub-arrays), and each sub-array may include plural source points (e.g., an air gun or a cluster, association of several air guns, etc.) provided along a Y direction as shown in FIG. 2. This source array 50 has better characteristics than existing source arrays and it is disclosed in patent application Ser. No. 13/468,589, filed on May 10, 2012, and assigned to the same assignee as the present application, the entire disclosure of which is incorporated herein by reference. The source array 50 may include three different sub-arrays 60a-c, each having a corresponding float 52a-c, respectively. From each float a plurality of source points 64 is suspended. However, different from the existing sources, note that source points 64 are suspended, from the same float, at two different depths, and the configuration of the source points attached to one float may be different from the configuration of the source points attached to another float. For example, FIG. 2 shows that sub-array 60a has the deeper source point behind the shallow source points along direction Y, while sub-array 60c has the deeper source point between the shallow source points along the Y direction.
However, to produce a seismic source as illustrated in FIG. 2 has its own challenges as explained next. A traditional source array 100 is illustrated in FIG. 3 and includes a float 102 from which multiple plates 104 are suspended at a given depth. The float 102 has a body that extends along a longitudinal axis (X). Cables 106 may be used to suspend the plates 104 from the float 102. Plural source points 108a to 108e form the given depth sub-array set 108. All these source points are suspended from the same float 102 via links 112 that substantially extend along a vertical axis (Z). The link 112 may include a chain, a rope and/or a cable. Each source point may have its own cables 114 (electrical, compressed air, data, etc.) for controlling and activating the source point (note that these cables are not shown for all the sources). The cables are protected by a rigid housing 115. Strength members 110 may be located between the plates 104 for maintaining the source's integrity when towed underwater.
Some of the source points may optionally be connected to each other by various means 116, e.g., rods, chains, cables, etc. A front portion of the plate 104 corresponding to the first source point 108e and its air gun may also be connected via a connection 118 to an umbilical 120 that may be connected to the vessel (not shown). Optionally, a link 122 may connect the float 102 to the umbilical 120. In one application, three or more such floats 102 and corresponding source points may form the source array 100.
For this same-depth source array, the housing 115 and the strength members 110 are substantially parallel to the float. However, if the source array 100 is configured to have source points 108a-e at different depths, as shown in FIG. 2, then a great amount of stress is exerted on those portions of the housing 115 and strength members 110 that make the transition from the shallow depth to the deep depth of the source array. In this case, those portions of the housing 115 may break or prevent the source points from achieving a desired depth. Thus, various modifications to the housing 115 need to be made in practice. However, any modification is disruptive because it requires research, feasibility studies, etc., and takes time to be implemented.
Therefore, it is desired to produce a new source array that overcomes these problems and achieves strong low-frequency energy, a smooth spectrum, and a reduced number of high-frequency ghost notches. In addition, it is desired that such a source can be easily changed from a first configuration (e.g., same depth for the source points) to a second configuration (e.g., different depths for the source points).